Don't extend Document... Just don't.

DontInheritFromDocument.cs

using Microsoft.Azure.Documents;
using Microsoft.Azure.Documents.Client;
using Microsoft.Azure.Documents.Linq;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    public class Program
    {
        static void Main(string[] args)
        {
            DocDbDocumentProof.Run().Wait();
        }
    }

    public class ExtendsDocument : Document
    {
        [JsonProperty("something")]
        public int Something { get; set; }

        [JsonProperty("partitionKey")]
        public string PartitionKey { get; set; }
    }

    public static class DocDbDocumentProof
    {
        public static async Task Run()
        {
            const string db = "<database-name>";
            const string collection = "<collection-name>";
            const string databaseUri = "<database-uri>";
            const string authKey = "<database-key>";

            var collectionUri = UriFactory.CreateDocumentCollectionUri(db, collection);

            var client = new DocumentClient(
                new Uri(databaseUri),
                authKey,
                new ConnectionPolicy
                {
                    ConnectionMode = ConnectionMode.Direct,
                    ConnectionProtocol = Protocol.Tcp,
                    RetryOptions = new RetryOptions
                    {
                        MaxRetryAttemptsOnThrottledRequests = 10,
                        MaxRetryWaitTimeInSeconds = 10
                    }
                });

            var doc = new ExtendsDocument
            {
                Id = "My unique Guid",
                Something = 10,
                PartitionKey = "My unique partition key"
            };
            await client.CreateDocumentAsync(collectionUri, doc);
            var results = client.CreateDocumentQuery<ExtendsDocument>(collectionUri)
                .Where(x => x.PartitionKey == doc.PartitionKey && x.Id == doc.Id)
                .AsDocumentQuery();

            IEnumerable<ExtendsDocument> allResults = new List<ExtendsDocument>();
            do
            {
                allResults = allResults.Concat(await results.ExecuteNextAsync<ExtendsDocument>());
            } while (results.HasMoreResults);

            var myDoc = allResults.FirstOrDefault();
            if (myDoc == null) return;

            Console.WriteLine(JsonConvert.SerializeObject(myDoc));
            myDoc.Something = 11;
            Console.WriteLine(JsonConvert.SerializeObject(myDoc));

            await client.DeleteDocumentAsync(myDoc.SelfLink, new RequestOptions
            {
                PartitionKey = new PartitionKey(myDoc.PartitionKey)
            }); // Clean up the mess

            Console.ReadLine();
            // You will see in console something like
            /*
            {"something":10,"partitionKey":"My unique partition key","id":"My unique Guid","_rid":"...","_self":"...","_ts":0,"_etag":"...","something":10,"partitionKey":"My unique partition key"}
            {"something":11,"partitionKey":"My unique partition key","id":"My unique Guid","_rid":"...","_self":"...","_ts":0,"_etag":"...","something":10,"partitionKey":"My unique partition key"}
            */
            // You may notice that the serialization of the object duplicates the 'something' and 'companyId' properties
            // And that the 2nd line changes the something to 11 successfully, but the 2nd occurance of it has the orignal 10 in it
        }
    }
}